refactor(iota-core): silence unused variable warning

[Thoralf-M]

Description of change

Silence unused variable warning

warning: unused variable: `ccp_digest`
    --> crates/iota-core/src/checkpoints/mod.rs:1107:22
     |
1107 |         if let Some((ccp_digest, ccp_effects)) = consensus_commit_prologue {
     |                      ^^^^^^^^^^ help: if this is intentional, prefix it with an underscore: `_ccp_digest`
     |
     = note: `#[warn(unused_variables)]` on by default

it's used when compiling not in release mode because of #[cfg(debug_assertions)]

Type of change

• Enhancement (a non-breaking change which adds functionality)

How the change has been tested

cargo check -r